Nanoporous surfaces of FeAl formed by vacancy clustering

Numerous thermal vacancies are frozen into FeAl B2-ordered alloy ribbons by a conventional rapid-solidification technique. Through heat treatment at 723 K, clustering of the supersaturated vacancies generates a large number of nanopores, particulary near surfaces, thus creating nanoporous surfaces. The nanoporous surface structure was confirmed by scanning electron microscopy and atomic force microscopy. The behavior of this vacancy clustering was examined by differential scanning calorimetry. An exothermic, irreversible peak, probably due to vacancy clustering, was observed around 800 K, giving an activation evergy of about 1.17 eV. The nanopore formation was also observed by in-situ heating experiments in a transmission electron microscope. The pores have specific morphology and crystallography with pore surfaces faceting toward {100} planes. These results suggest that the vacancy clustering is a unique process which enables us to efficiently make nanoporous surfaces.